The invention relates to a method of producing a metallic coating on an object emerging from a bath of molten metal. The invention also relates to a device applying the said method.
It has a particularly interesting application in the field of the manufacture of electrode wire for spark erosion. For this purpose, first a metallic coating is made, in zinc for example, on a metallic wire, of copper or steel for example, then the coated wire is placed in a heat-treatment furnace so as to obtain diffusion of the zinc into the metal wire.
It is also possible to make a coating of tin on a core of steel or of copper, and the product obtained is then intended to undergo drawing operations.
The invention can also find applications in other fields such as the production of a metallic coating for protecting a non metallic core, for example an optical fibre.
The general principle of manufacture of electrode wire for spark erosion is described extensively in the prior art, in particular in documents U.S. Pat. No. 4,169,426 and EP-A-0 811 701, in which a conducting wire passes vertically through a bath of molten metal and is then subjected to treatments for the purpose of being drawn. The complex and costly process described in document U.S. Pat. No. 4,169,426 relates to a pretreatment for cleaning metal wire before the latter passes through the bath of molten metal and undergoes rapid cooling. Document EP-A-0 811 701 describes two electrodes in contact with the metal wire, respectively upstream and downstream from the bath of molten metal, so that the part of the metal wire between the two electrodes is heated by the Joule effect, by passing a current through these electrodes.
One of the principal characteristics in the production of a coating is the thickness of the outer layer obtained. Theoretical results relating coating thickness to the speed of travel of the metal wire and to the hydrodynamic properties of the molten metal were established in particular by L. Landau and B. Levich in an article in Acta Physicochimica U.R.S.S. Vol. XVII, No. 1-2, 1942: xe2x80x9cDragging of a Liquid by a Moving Platexe2x80x9d. This article gives an equation relating, in first order, the coating thicknessxe2x80x94which is assumed constantxe2x80x94to a capillary number that is a function of the hydrodynamic properties of the molten metal, provided that the molten metal is a liquid that wets perfectly and the object being coated is a plate.
Now, on the basis of the aforementioned theoretical results, the thickness obtained is often too great for coating applications in which a fine thickness is desired. Accordingly, various forms of wiping, i.e. of reducing the thickness of the coating formed, have been proposed, such as techniques of pneumatic wiping (action of air knives forming a back-pressure on the free surface of the metallurgical product emerging from the liquid bath), techniques of mechanical wiping (action of rollers that xe2x80x9clickxe2x80x9d the metallurgical product by means of asbestos pads) and finally, techniques of magnetic wiping, the present invention belonging to this last-mentioned category.
The magnetic wiping techniques make use of the Lorentz forces that are generated in the coating liquid by a magnetic field, static or alternating, fixed or sliding. The action of a magnetic field on a liquid metal is known and is described in particular in document U.S. Pat. No. 4,324,266. This document discloses a device for accomplishing the confinement of a jet of liquid metal by creating an overpressure by means of a coil encircling the jet and carrying an alternating current whose frequency is below a given value. In general, many techniques of magnetic wiping are included in the state of the art, in particular patent EP 0 720 663 B1 of the present applicant, in which an inductor, arranged around an exit channel of the bath of molten metal, produces a transverse, alternating electromagnetic field of quite low frequency, and sliding, the movement of the galvanized product taking place along a horizontal axis. The device thus embodied makes it possible to determine the conditions for which the Couette lengths associated with the flow of the coating liquid respectively in the container and in its exit channel remain below critical values, above which the flows become decidedly turbulent. These conditions require accurate dimensioning in the vessel containing the liquid metal and make it possible, in the case of horizontal drainage, to keep the molten metal inside the exit channel. The thickness is controlled according to a formula similar to that employed in the hydrodynamic model of Landau and Levich, the references for which were cited above. However, the method described in this document EP 0 720 663 B1 cannot relate to products of small thickness, as the design of the inductor means that its air gap is too large for the sliding field created by the said inductor to be able to act effectively on the said products.
Document U.S. Pat. No. 4,228,200 describes a method of controlling the metallic coating on a wire emerging vertically from a bath of molten metal. The thickness is controlled by means of a single-coil device creating a fixed, alternating electromagnetic field of very low frequency, applied at the point of exit or below the point of exit of the wire. The electromagnetic field thus created expels the molten metal from the zone of highest flux density towards zones with a lower flux density. Coating thickness is adjusted by altering the amplitude of the electromagnetic forces exerted by the field generated by the electromagnetic device. However, as can be seen in FIGS. 3A and 3B of document U.S. Pat. No. 4,228,200, the device saturates for a frequency above 300 Hz for example. The magnetic field created no longer exerts an influence on the thickness of the coating. In addition, this saturation must be strongly dependent on the type of metal used, since each metal has a different saturation level.
There is a known method of magnetic wiping, developed by M. Malmendier, J-F. Noville and S. Wilmotte of the Metallurgical Research Centre (Centre de Recherches Mxc3xa9tallurgiques, CRM) of Liege, and disclosed in the xe2x80x9cConference Proceedingsxe2x80x9d with the title xe2x80x9cImprovement of control of the zinc loading in the hot-dip galvanizing processxe2x80x9d, pages 407-412, 27-29 May 1997. This method employs a magnetic field created by means of an alternating current, acting on the thickness of the coating already formed. However, the said method requires the use of high power, and involves an excessive temperature rise of the coating.
The present invention aims to remedy the aforementioned drawbacks and relates to a method of making a coating in which the coating thickness is controlled accurately, by taking into account all of the parameters involved in the production of the said coating.
Another object of the invention is the production of a coating of small thickness, typically of the order of a micrometer on small objects, with low energy consumption and limiting the temperature rise of the coating.
Yet another object of the present invention is a device in which the vessel containing the bath of molten metal is suitably dimensioned so as to permit efficient control of coating thickness regardless of the type of drainage of the object (vertical, slanting or horizontal).
The aforementioned aims are achieved with a method of producing a metallic coating on an object emerging from a bath of molten metal, in which a magnetic field is created near the point of exit of the object. According to the invention, the object leaves the bath of molten metal through an exit channel containing a meniscus of the said bath of molten metal, and the thickness of the metallic coating is controlled as a function of a second derivative of the curve of the meniscus and of a capillary number Ca representing the ratio between the viscous forces of the molten metal and the forces of surface tension at the surface of the molten metal.
This characteristic can be represented in the form of an equation:                     e        0            ·              ϕ        zz              =          1.3      ⁢              Ca                  2          3                      ,
e0 is the thickness, xcfx86zz is the second derivative of the meniscus and z is the axis of travel.
The object to be coated can advantageously be a linear product of constant cross-section such as a wire or thread, e.g. a metal wire or an optical fibre, or a plate. For a plate of small thickness, the shape of the meniscus on the large sides is taken into account.
With such a method, the invention offers an advantage relative to the documents of the prior art, as it expresses the thickness as a function of the physical elements represented in the second derivative and in the number Ca, which is explained below.
The properties of the coating, especially its thickness, result from competition between mainly four types of forces:
the forces of gravity, proportional to xcfx81g, where xcfx81 is the density of the molten metal, and g is the acceleration of gravity;
the forces of viscosity, proportional to xcexcV, where xcexc is the dynamic viscosity of the molten metal, and V is the velocity, characterizing the movement of the object relative to the molten metal;
the forces of surface tension, proportional to xcex3, where xcex3 is the interfacial tension between the molten metal and the air; and
the repulsive forces of electromagnetic origin between an inductor, through which an alternating current is passing, and the molten metal, these forces being proportional to                     C        f            ⁢              B        0        2                    2      ⁢              μ        0              ,
where B0 is the magnetic field, xcexc0 is the magnetic permeability of the molten metal, and Cf is a coefficient such that             C      f        =          1      -                        1          -                      ⅇ                          -                                                R                  ω                                                                                          R            ω                                ,
with a screening parameter Rxcfx89=xcexc"sgr"xcfx89l2, "sgr" is the conductivity of the metal, xcfx89 is the angular frequency, and 1 is a dimension that is characteristic of the geometry, such as the radius xe2x80x9crxe2x80x9d for a wire and the capillary length xe2x80x9caxe2x80x9d for a plate.
The capillary number Ca represents the ratio between the forces of viscosity and the forces of surface tension:   Ca  =                    μ        ⁢                  xe2x80x83                ⁢        V            γ        .  
According to one embodiment of the invention, during vertical drainage upwards, the exit channel is dimensioned so as to keep the meniscus of the molten metal in conditions close to capillary-gravitational equilibrium in the magnetic field. Under these conditions, the second derivative of the curve of the said meniscus is a function of an electromagnetic forming parameter K representing the ratio between the forces of surface tension and the forces due to the effect of electromagnetic forming:   K  =                    2        ⁢                  μ          0                ⁢        γ                              C          f                ⁢                  B          0          2                ⁢        I              .  
In this case of vertical drainage upwards, and for a plate, the expression for the second derivative can be as follows:       ϕ    zz    =            1              aK        ⁢                  xe2x80x83                ⁢                  cos          3                ⁢        θ        ⁢                  xe2x80x83                ⁢        e              ⁢                  1        +                  2          ⁢                                    K              2                        ⁡                          (                              1                -                                  sin                  ⁢                                      xe2x80x83                                    ⁢                  θ                  ⁢                                      xe2x80x83                                    ⁢                  e                                            )                                          
where xe2x80x9caxe2x80x9d is the capillary length (known value)       a    =                  γ        pg              ,
and xcex8e is the acute angle at the intersection of the apex of the meniscus with the wall of the object to be coated.
In the case of a wire, the expression for the second derivative can be as follows:       ϕ    zz    =                    Bd        ·                  λ          2                    +              1        K            -              cos        ⁢                  xe2x80x83                ⁢        θ        ⁢                  xe2x80x83                ⁢        e                    1.3      *      r      *              cos        3            ⁢      θ      ⁢              xe2x80x83            ⁢      e      
r is the radius of the wire; xcex2 is such that r*xcex2 is equal to the height of the meniscus 12, xcex2 is preferably obtained by numerical calculation; and Bd is a Bond number representing the ratio between the forces of gravity and the forces of surface tension:   Bd  =                    pgr        2            γ        .  
It is thus possible for the coating thickness e0 to be determined accurately.
These equations have been established in the case of vertical drainage upwards and provided we are close to capillary-gravitational equilibrium in an electromagnetic field in which the forces of gravity and of electromagnetic forming are compensated by the forces of surface tension.
The exit channel can be constructed in such a way that the annular distance is of the order of the height of the meniscus, the annular distance being the distance between the inside wall of the exit channel and the metallic coating formed outside of the meniscus. In the case of a plate, the height l2 of the meniscus can be obtained from the following expression:       l    2    =            a      k        ⁢          (                                    1            +                          2              ⁢                                                K                  2                                ⁡                                  (                                      1                    -                                          sin                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        e                                                                              )                                                                    -        1            )      
According to a variant of the invention, during vertical drainage downwards, the second derivative of the curve of the said meniscus is a function of:
the ratio between the average thickness of the said object and the opening of the exit channel; and
the ratio between the Alfen rate and the rate of drainage of the said object.
The Alfen rate UA is given by the expression:       U    A    =                                          C            f                          ·                  B          0                                                  μ            0                    ⁢          ρ                      .  
In this first variant according to the invention, an expression of the second derivative of the curve of the meniscus in the case of a wire for example can be as follows:       ϕ    zz    =            1      R1        ⁡          [              2        +                                            (                              R1                RO                            )                        4                    ·                      (                          1              +                                                U                  A                  2                                                  α                  ⁢                                      xe2x80x83                                    ⁢                                      V                    0                    2                                                                        )                              ]      
where R1 is the radius of the wire, R0 is the radius of the opening of the exit channel, V0 is the velocity of travel of the wire, and xcex1 is a term reflecting the influence of the Couette flow, equal to:             1      2        ⁡          [                                    1            -                                          (                                  R1                  R0                                )                            2                                            1            ⁢            n            ⁢                          xe2x80x83                        ⁢                          1                              (                                  R1                  R0                                )                                                    -                  2          ⁢                                    (                              R1                R0                            )                        2                              ]        .
According to one embodiment of the invention, the exit channel is constructed in such a way that the ratio between the average thickness of the said object and the opening of the exit channel is greater than or equal to 0.8 so as not to have intense fields.
In the case of a circular wire, the average thickness is the diameter. In the case of a non-circular wire, the average thickness is an estimated value.
A special feature of the present invention is that it avoids the influence of gravity. Thus, in contrast to some methods of the prior art that create a magnetic field acting in the coating layer already formed, the magnetic field according to the invention acts directly on the meniscus.
According to the invention, the magnetic field can be alternating and steady-state, and it can be created advantageously by means of a flat inductor. An inductor of the xe2x80x9cPancakexe2x80x9d type can be used.
The invention is thus remarkable in that the magnetic field created only acts upon a small height of the molten metal forming the coating. Thus, the rise in temperature of the coating due to the magnetic field is advantageously small relative, for example, to the method proposed by the Metallurgical Research Centre of Liege, cited above.
Thus, for comparison, using the formulae established by the Metallurgical Research Centre, under the following conditions:
for an intended thickness of 10 xcexcm,
for a line speed of 60 m/min,
in the case of complete wetting, xcex8e=0, we obtain a magnetic field intensity B0=0.71 T and a temperature rise of xcex94T ≅100xc2x0 C. by the method of the Metallurgical Research Centre.
With the same conditions as above, the method according to the invention gives: B0xe2x88x920.078 T and xcex94T≅7xc2x0 C.
The magnetic field is preferably created by means of an alternating current whose frequency is such that the ratio between the capillary length and the magnetic skin thickness in the metallic coating is greater than or equal to 3.
According to another variant of the invention, in the case of horizontal drainage with an exit channel containing a meniscus obtained by applying a sliding field in the bath of molten metal, the second derivative of the curve of the meniscus is a function of a Bond number Bd representing the ratio between the forces of gravity and the forces of surface tension:   Bd  =                    ρ        ⁢                  xe2x80x83                ⁢                  gl          2                    γ        .  
This second variant makes advantageous use of the teaching contained in document EP 0 720 663 B1.
According to an advantageous characteristic of the invention, means are employed for pressure or electromagnetic pumping of the molten metal to maintain the height of the meniscus in the exit channel, making it possible to compensate the continuous consumption of molten metal in production of the said coating.
The invention also relates to a device for producing a metallic coating on an object emerging from a bath of molten metal. The device includes means for creating a magnetic field near the exit point of the said object. The device can include an exit channel containing a meniscus of the said bath of molten metal, as well as means for adjusting the thickness of the metallic coating as a function of a second derivative of the curve of the meniscus and of a capillary number Ca representing the ratio between the viscous forces of the molten metal and the forces of surface tension at the surface of the molten metal.